VOR (Very High Frequency OMNI Ranging) ground stations transmit a signal to airplanes or other vehicles conveying magnetic bearing information from the station. This bearing information is conveyed as the phase difference between two 30 hertz modulation signal components. One of these components, the variable phase, is imposed upon the radio frequency carrier as amplitude modulation. The other component, the reference phase, is transmitted by frequency modulating a 9,960 hertz subcarrier at a 30 hertz rate and imposing the subcarrier upon the radio frequency carrier by amplitude modulation.
In a standard VOR receiver, an FM discriminator recovers the reference phase signal from the subcarrier. The recovered 30 hertz reference signal is passed through a bandpass filter, while another bandpass filter selects the 30 hertz variable phase signal which was imposed as amplitude modulation on the carrier. These two filters extract the reference and variable phase signals from noise and other modulation components. Bearing information from the ground station is then derived by measuring the phase difference between the two 30 hertz signals at the bandpass filter outputs.
The phase measurement may be accomplished by measuring the time interval between similar slope zero crossings of the two 30 hertz signals and dividing this time by the period of one of the signals. The quotient may be interpreted as a fraction of 360 degrees. Such a measurement is independent of signal frequency errors.
Conventional prior art circuits for making the VOR phase measurement require at least one initial service adjustment to correct differential phase shift between the two bandpass filters. Some prior art circuits employ an additional initial adjustment to correct for differential DC offset voltages between the two channels. Aging of components then typically requires further adjustments.
The typical method of making the phase measurement in the prior art involves squaring the leading and trailing edges of both 30 hertz sine-wave signals and making time measurements between the edges of the resultant rectangular waveforms. A DC offset voltage anywhere in the system results in an improper zero crossing detection with a resulting displacement in time of the edge of the rectangular waveform from the mathematical zero crossing of the original sine wave. If the offsets are not identical for the two channels, reference and variable, the resulting unequal time displacements will result in bearing error.
Component parameter drift due to aging or temperature variations further provides errors by altering both differential phase shift between the two channels and also DC offsets. The two alterations produce bearing errors. Conventional prior art circuits, therefore, must employ very stable components to maintain acceptable operational accuracy.
It is therefore an object of the present invention to provide a circuit which provides measurement of the phase without having a requirement for extremely stable components.
Another object of the present invention is to provide a circuit which accomplishes phase measurement while eliminating the requirement for service adjustments.